CSipSimple message to SMS with chan_dongle


I’m running a standard installtion of Freepbx 13, Asterisk 13.12.2 on debian 8 with the chan_dongle module. Voice calls, receiving SMS and forwarding them to another number all work. Now I’d like to be able to send SMS from CSipSimple. I followed several guides but nothing worked. Can someone direct me to a working guide?